[pdonis]

Most words are "flexible" and you have to take them in context to choose which of multiple possible meanings is most appropriate. In the case of explosive weapons, "on" usually means that the object of the preposition is the point of impact, as in "dropping bombs on target X".

[IntrepidWorm]

TL:DR; The phrase "A bird dropped a large splat on my head" generally implies a much more upsetting prospect than simply "A bird dropped a large splat in my general vicinity."